i) Field of the Invention
The present invention relates to manufacturing a projection cathode-ray tube with a uniform optical multiple layered interference film for use in a projection television set having a superior white uniformity of a reproduced image for projecting an enlarged image of an image reproduced onto a fluorescent surface or plate of the cathode-ray tube on a screen located in front of the cathode-ray tube through a projection lens arranged between the fluorescent plate of the cathode-ray tube and the screen.
ii) Description of the Related Arts
Conventionally, a method for improving a beam-condensing rate when a light beam emitted by a projection cathode-ray tube for each primary color is introduced into a projection lens unit in a projection television set, is disclosed in U.S. Pat. No. 4,642,695.
In a usual cathode-ray tube, a light beam emitted from a fluorescent surface or plate is almost perfectly diffused light. On the other hand, in a projection television set, only the light components within a divergent angle of .+-.30.degree. of the light beam emitted from the fluorescent plate of a cathode-ray tube can be introduced into a method of manufacturing a projection lens unit and be used effectively. The other light components become useless light. These useless light components bring about various drawbacks. For example, the useless light components are reflected by the lens-barrel of the projection lens unit to become stray light. In this respect, in the aforementioned U.S. Pat. No. 4,642,695, more than 30% of all luminous flux emitted from an emission point in a fluorescent plate is concentrated into a conical body within a divergent angle of .+-.30.degree. in order to largely improve the image brightness on the screen in the projection television set.
Further, in Japanese patent laid-open No. Sho 60-257043, as one embodiment of the invention in U.S. Pat. No. 4,642,695, a cathode-ray tube having an optical multiple layered interference film composed of a plurality of laminate layers formed by alternately laminating high and low refraction materials between a glass face panel and a fluorescent plate is disclosed. In this instance, one embodiment of a six-layer optical multiple layered interference film using tantalum pentoxide Ta.sub.2 O.sub.5 and silicon dioxide SiO.sub.2 as the respective high and low refraction materials is described.
In FIG. 1, there is shown a conventional projection cathode-ray tube 1 with an optical multiple layered interference film. In the projection cathode-ray tube 1 with the optical multiple layered interference film, a glass face panel 2 and a glass funnel 3 constitute a vacuum vessel. An electron gun 5 for emitting an electron-beam for exciting a fluorescent plate is sealed in a neck portion 4 of the glass funnel 3. On the inner surface of the glass face panel 2, there are successively provided an optical multiple layered interference film 6 for giving directivity to the light radiation from the fluorescent plate to concentrate the light radiation, a fluorescent body layer 7 to be luminous due to the excitement by the electron-beam from the electron gun 5, and a metal back film 8 For reflecting the luminescence of the fluorescent body layer 7 forwards with as high an efficiency as possible to increase the light output. The optical multiple layered interference film 6, the fluorescent body layer 7 and the metal back film 8 constitute the fluorescent plate.
In FIG. 2, there is shown a part of the fluorescent plate and the glass face panel 2 shown in FIG. 1. In this case, the optical multiple layered interference film 6 has the structure of a six-layer laminate film formed by alternately laminating the high and low refraction materials H and L of tantalum pentoxide Ta.sub.2 O.sub.5 and silicon dioxide SiO.sub.2, respectively.
As is apparent from FIG. 1, the inner surface of the glass face panel 2 of the projection cathode-ray tube 1 with the optical multiple layered interference film is usually formed with a curved surface and the curvature radius R along the diagonal screen axis is often selected to be approximately 350 mm. By utilizing this inside convex curved surface, the radiation from the peripheral portion of the fluorescent plate of the projection cathode-ray tube can be introduced into the projection lens unit with high efficiency, and a uniform brightness of the projected image can be ensured. Further, the glass face panel 2 is provided with a skirt portion 9 of a side wall portion parallel with the axis direction, and the skirt portion 9 is connected to the end surface of the glass funnel 3 by a glass frit 10.
The skirt portion 9 is essential for the following two reasons. That is, firstly, when no skirt portion 9 is provided, a large vacuum stress exists in the vicinity of the junction surface by the glass frit 10, and the mechanical strength of the projection cathode-ray tube 1 becomes very weak. Secondly, when there is no skirt portion 9 in the manufacturing process of the fluorescent plate or the like, handling becomes very difficult.
In FIG. 3, there is shown a vacuum evaporator 20 for forming the optical multiple layered interference film 6 on the inner surface of the glass face panel 2. In a bell jar 20 as a vacuum vessel, the gas is discharged by a vacuum pump 22, and the inside of the bell jar 20 is kept in a very high vacuum state. Within the bell jar 20, the glass face panels 2 having the optical multiple layered interference film formed on them are mounted on a support plate 24 supported by support legs 23. The glass face panels 2 are heated over 350.degree. C. by heaters 25. By heating the evaporation surfaces of the glass face panels 2, a stable and strong optical multiple layered interference film can be formed.
A fixed evaporation source 26 of tantalum pentoxide Ta.sub.2 O.sub.5 and a fixed evaporation source 27 of silicon dioxide SiO.sub.2, are heated for evaporating by the electron-beam. When the inside of the bell jar 21 reaches a predetermined degree of vacuum and the temperature of the glass face panels 2 within the bell jar 21 reaches a predetermined value, the evaporation source 26 of tantalum pentoxide Ta.sub.2 O.sub.5 is actuated by the electron-beam, and tantalum pentoxide Ta.sub.2 O.sub.5 vapor is emitted from the evaporation source 26 in the bell jar 21 and adheres to the inner surface of the glass face panels 2 to form a vacuum evaporation film of tantalum pentoxide Ta.sub.2 O.sub.5 thereon. At this time, in order to obtain a stable vacuum evaporation film of the tantalum pentoxide Ta.sub.2 O.sub.5, a certain amount of oxygen gas O.sub.2 can be introduced in the bell jar 21 during the evaporation process.
In order to obtain the necessary optical characteristics of the optical multiple layered interference film, the thickness of the evaporation film should be strictly controlled. Therefore, the thickness of the evaporation film adhered to an evaporation film thickness monitor plate 29 is measured by an evaporation film thickness controller 28 to carry out a film thickness control. When the film thickness of the tantalum pentoxide Ta.sub.2 O.sub.5 of the first layer has reached a predetermined thickness, the action of the evaporation source 26 is stopped. Then, the evaporation source 27 of silicon dioxide SiO.sub.2 is actuated to form a vacuum evaporation film of silicon dioxide SiO.sub.2 being the second layer having a predetermined thickness on the evaporation film of the tantalum pentoxide Ta.sub.2 O.sub.5 of the first layer in the same manner as the evaporation source 26 of the tantalum pentoxide Ta.sub.2 O.sub.5. These operations are repeated to form the optical multiple layered interference film 6 having six layers alternately laminated with the high refraction material of tantalum pentoxide Ta.sub.2 O.sub.5 and the low refraction material of silicon dioxide SiO.sub.2 on the inner surface of the glass face panel 2.
In the conventional projection cathode-ray tube with the optical multiple layered interference film, as described above, the thickness of the optical multiple layered interference film 6 formed on the inner surface of the glass face panel 2 is uneven at the periphery, in particular, the diagonal peripheral portions of the glass face panel 2, and the optical characteristics of the optical multiple layered interference film 6 are insufficient in those portions. As a result, the desired luminescence characteristics of the projection cathode-ray tube 1 with the optical multiple layered interference film cannot be obtained in the diagonal peripheral portions of the fluorescent plate, and hence an abnormality is caused in the white uniformity of the image reproduced in the projection television set.
The cause of the abnormality in the white uniformity will now be described in detail in connection with FIG. 4. In FIG. 4, there are shown cross sections AA, BB and CC of the glass face panel 2, taken along the lines in the directions of a long axis diameter A, a short axis diameter B and a diagonal axis diameter C, respectively. Relating to the optical multiple layered interference film 6 formed on the inner surface of the glass face panel 2, the thickness becomes fairly thin particularly in the diagonal peripheral portions 30 compared with the central portion of the glass face panel 2. This is caused by obstructing the uniform adhesion of the refraction material vapor onto the inner surface of the glass face panel 2 under the influence of the skirts 9 such as long side skirts 9a of long sides 2a and short side skirts 9b of short sides 2b of the glass face panel 2 in the diagonal peripheral portions 30 when the vacuum evaporation of the high and low refraction materials onto the glass face panel 2 is carried out.
In FIG. 5, there is shown one example of the spectral transmittance property of the optical multiple layered interference film 6 formed on the inner surface of the glass face panel 2 on the normal-incidence of the light. This spectral transmittance property is represented by a value of a wavelength .lambda..sub.50 at a point exhibiting 50% of spectral transmittance. A curve (I) shows the spectral transmittance at the central portion of the glass face panel 2, obtaining .lambda..sub.50 =490 nm. Further, a curve (II) shows the spectral transmittance at the diagonal portion 30 of the glass face panel 2. In this case, the evaporation film becomes uneven due to the influence of the skirts 9 to obtain .lambda..sub.50 =470 nm. Hence, the wavelength difference .DELTA..lambda..sub.50 between the wavelengths at the central and the diagonal portions is 20 nm. The limit value of the wavelength difference .DELTA..lambda..sub.50 is considered to be approximately 10 nm from various experiment results. As a result, when .DELTA..lambda..sub.50 becomes 20 nm, the desired luminescence characteristics of the projection cathode-ray tube 1 with the optical multiple layered interference film cannot be obtained in the diagonal peripheral portions of the fluorescent plate, and hence the abnormality is caused in the white uniformity of the image reproduced in the projection television set.